Monday, January 28, 2013

Windswept, Soggy Midweek Ahead

It's going to be a wild time of things over the next 48 to 60 hours around the region as a strong storm system organizes across the Midwest and pushes through the country. This storm will not only bring a quick shot of warmth to the Delaware Valley but it will also pack plenty of energy along for the ride. The potential result is a windswept, soggy rain looks to be in the offing for the Delaware Valley on Wednesday afternoon and evening.

The Storm Prediction Center yesterday highlighted the possibility of severe criteria winds on Wednesday night across Pennsylvania as the cold front approaches and crosses the region. These won't be severe thunderstorms in the classic sense but a winter squall line of windswept moderate and heavy rainfall that develops along the cold front and pushes east as the front approaches the region Wednesday evening. They since scaled back on this potential in today's outlook but the strong wind threat along and ahead of the cold front still remains.

Computer modeling shows a very robust, potent low level jet ahead of the storm system. Lots of wind energy, with the GFS suggesting winds just a few thousand feet above ground could exceed 65 kts (which is 75 mph). Lots of wind energy for this storm system to tap into and transport to the ground. Given the strong southerly push of wind aloft and at the surface, moisture will transport north with this storm system and we'll see areas of rain break out ahead of the front on Wednesday midday or Wednesday afternoon. This line will push east on Wednesday night, probably crossing the region after 7 or 8 PM. Timing still to be determined but it looks like a Wednesday evening/night threat.

The models are indicating the potential for at least an inch of rain for much of the region. Computer modeling waffles on who gets the heaviest rains, with yesterday's GFS runs showing north/west and last night's GFS runs showing south/east possibly picking up two inches of rain, with almost everyone else getting an inch of rain regardless of who gets the heaviest. Since this storm circulation tracks to our west it is a rainstorm for the region -- no snow to be had around here. It will help tap into the cold air that retreated into Canada and will reinforce that regime for the beginning of February.

The heavy rain threat is something to keep note of -- the ground in many spots will still be frozen despite a short-term thaw through Wednesday night. Heavy rains over frozen ground yield high runoff rates so stream flooding is certainly possible north and west of the city. Keep an eye on the modeling trends over the next 48 hours if you live near some of the more flood-happy and usual suspect streams in Eastern Pennsylvania.